Fun fact: Part of the Roman Empire's strength was its tolerance and integration of other cultures instead of trying to impose their own. Their various Italian subjects were "allies" in name and bound by pacts of mutual assistance, but otherwise largely left to govern themselves as long as they paid their tithes.
The prospect of becoming Roman Citizens in exchange for military service enticed them to serve as auxiliary units. These units were also tightly integrated with the Roman troops, often leading to a gradual blending of cultures, a layering of identities rather than exclusively choosing them.
Sure, this wasn't some altruistic act of goodwill so much as political strategy to maximise their power, but it worked. Part of the fall was the fracturing of loyalties because later Emperors abandoned that policy and it's just not as appealing to send your sons off to war for an empire that will only ever treat you as second class subjects.
Of course, the whole thing is more complicated than I could convey in a single comment, but check out this ancient historian's blog series on "Who were the Romans?" and "Decline and Fall?".